The present invention relates to data symbol timing recovery techniques for digital communication systems using a burst mode communication format.
Existing cellular mobile communication systems are generally regional systems that require numerous antennas scattered over the system's coverage area. The cost of locating and operating the antennas over a large coverage area is expensive and, accordingly, mobile communication services are often limited to metropolitan areas or along heavily traveled highways. Mobile communication services generally are not available in rural areas and along remote highways.
Satellite-based mobile communication systems are being developed for providing service on a global basis and to remote areas. A satellite-based communication system 1, as shown in FIG. 1, is able to cover a relatively large geographical area because the satellite's antennas may be configured to cover areas as large as a continent.
Satellite-based mobile communication systems are subject to Rician channel fading. Rician channel fading results from a combination of fading due to atmospheric effects and of Rayleigh fading which is inherent in mobile communication systems. For various reasons, including those relating to Rician fading, digital methods are generally implemented in satellite-based mobile communication systems.
In a typical implementation, an audio voice signal is converted to a digital signal and transmitted from a mobile station 2 through the earth's atmosphere 3 such that a portion of the signal is captured by a receiver on a satellite 4. The satellite relays the digital signal to another mobile station 2 or to a ground-based station 5 for connection with land-based telephone lines. Because an audio voice signal has a relatively small bandwidth, several signals are often frequency or time multiplexed within a larger bandwidth of the satellite's communications channel. Time multiplexing, for example, involves sending the digital audio signals as relatively short data bursts.
The mobile station, and in some systems the satellite, must decode the digital signal after it is transmitted across the data channel. Transmission through the channel causes the digital symbols or pulses to have rounded corners due to the channel's limited bandwidth. Efficient reception of the digital signal is achieved if the digital pulses are sampled at the pulse's maximum value which is generally centered between the rounded corners.
Many valuable techniques are available for determining the timing of the symbol maxima for a modulated digital data signal. However, within the available power limitations of a satellite-based mobile communication system, these existing techniques often are not precise when recovering the symbol timing for a burst digital signal that is communicated over a channel which is subject to Rician fading and that has a relatively short burst time in comparison with the time between bursts.
Accordingly, there is exist a need for a mobile communication station that efficiently recovers the symbol timing of a burst of digital data within limited available power conditions. The present invention satisfies these needs.